1. Field of the Invention
The present invention relates to phosphors and a light emitting device using the same, and more particularly, to phosphors that have high luminous efficiency at desired wavelengths and good light output stability, and a light emitting device using the same.
2. Description of the Related Art
In general, light emitting devices (LEDs) emit light close to monochromatic light while other light emitting devices, for example, incandescent lamps emit light with a broad emission spectrum. Since the individual LEDs have different energies generated by combination of electrons and holes, the LEDs emit red light, green light, blue light, yellow-orange light, and yellow light according to characteristics thereof.
In recent years, LEDs that emit white light or produce a plurality of colors have been developed. A method of manufacturing a white LED among the LEDs includes a method of combining LED chips to produce white or a method of combining an LED chip emitting light of a predetermined color with phosphors fluorescing and emitting light of predetermined colors. The white LED, which has been widely used, is manufactured according to the latter.
For example, a white LED package can be manufactured by sealing a blue LED chip with molding resin in which a yellow phosphor is dispersed. When the blue LED chip generates light with a wavelength range of 460 nm, the yellow phosphor that absorbs the generated light emits light with a wavelength of 545 nm. Further, the light generated from the blue chip and the light from the yellow phosphor that have different wavelength ranges from each other are mixed to output white light.
White LEDs are being used as backlights of liquid crystal displays instead of general small lamps or fluorescent lamps. As discussed by S. Nakamura “The Blue Laser Diode,” Springer, Chapter 10.4, pp. 216-221, 1977, white LEDs can be manufactured by forming a ceramic phosphor layer on the output surface of a blue LED.
In the related art, an LED having InGaN single quantum wells is used as the blue LED, and cerium-doped yttrium aluminum garnet (YAG:Ce), Y3Al5O12:Ce3+ is used as a phosphor. Blue light emitted by the blue LED excites the phosphor such that the phosphor emits yellow light.
The blue light emitted by the blue LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. An observer (viewer) perceives the mixture of blue and yellow light as white light. The blue LED emits light (blue light) at a wavelength ranging from 420 to 480 nm. When the blue light is combined with the yellow phosphor, white light having a color temperature of 6000 to 8000 K and a color rending index (CRI) of approximately 77 is generated.
Further, the blue LED can generate white light by being combined with a phosphor converting blue light into red light and a phosphor converting blue light into green light. Appropriate phosphors need to have high excitation efficiency within a range of 420 to 480 nm and a wide chromaticity zone. Therefore, efforts have been made to seek red, orange, and yellow phosphors that are combined with electroluminescent devices to change the light emission or the chromaticity zone.
Most of the white LED displays in commercial use operate by converting a part of light emitted from the blue LED into yellow light, or red and green light by using phosphors. Further, the part of the blue light from the LED is transmitted through the phosphor and mixed with yellow or red and green phosphor emission to thereby generate white light that can be perceived. Many researchers have carried out research into phosphors and obtained various results. These results are shown in the following cited references. The disclosures of the cited references are expressly incorporated herein by reference.
Novel phosphors useful in the manufacture of white light emitting diodes are disclosed in U.S. Patent Publication No. 2005-0023963. The phosphors provided by the invention are described by the formulae: MA2(SxSey)4:B and/or M2A4(SxSey)7:B in which x, and y are each independently any value between 0 and 1, including 0 and 1 subject to the proviso that the sum of x and y is equal to any number in the range of between about 0.75 and about 1.25; M is at least one of Be, Mg, Ca, Sr, Ba, Zn; A is at least one of Al, Ga, In, Y, La, and Gd; and wherein the activator(s), B, comprises one or more element selected from the group consisting of: Eu, Ce, Cu, Ag, Al, Tb, Cl, Br, F, I, Mg, Pr, K, Na, and Mn, including mixtures comprising any two, any three, any four, any five, any six, any seven, or more of these elements in any proportion, and wherein the elements in these mixtures may each independently be present in any amount between 0.0001% and about 10% in mole percent based on the total molar weight of said composition.
A method of forming high efficiency emission alkali metal activated gallium sulfide phosphors by including an excess of 1-7% of gallium is disclosed in U.S. Pat. No. 6,544,438. A solution of a soluble gallium salt is added to a sulfate precipitate of an alkali metal sulfate and firing in hydrogen sulfide atmosphere at temperatures up to about 900° C.
In U.S. Pat. No. 6,417,019, a method of fabricating a light emitting device includes providing a light emitting diode that emits primary light, and locating proximate to the light emitting diode a (Sr1-u-v-x Mgu Cav Bax) (Ga2-y-z Aly Inz S4):Eu2+ phosphor material capable of absorbing at least a portion of the primary light and emitting secondary light having a wavelength longer than a wavelength of the primary light. The composition of the phosphor material can be selected to determine the wavelengths of the secondary light. In one embodiment, the light emitting device includes the phosphor material dispersed as phosphor particles in another material disposed around the light emitting diode. In another embodiment, the light emitting device includes the phosphor material deposited as a phosphor film on at least one surface of the light emitting diode.
A phosphor and a method of deposition are disclosed in U.S. Pat. No. 6,919,682. The phosphor comprises a composition of the formula M′aBa1-aM″2M′″4:RE, where M′ is at least one element selected from magnesium and calcium, M″ is at least one element selected from aluminum, gallium and indium, M″′ is at least one element selected from sulphur, selenium and tellurium, RE is at least one rare earth element, especially europium or cerium, and 0<a<1.
According to various different references including the above cited references, phosphors having new compositions or improved characteristics of known materials have been proposed. However, fluorescent materials currently being used to convert origin ultraviolet (UV), blue, or green light do not satisfy luminous efficiency and light output stability that are required when the fluorescent materials are used in LEDs.
In terms of these aspects, there has been a need for an LED that emits white light by using fluorescent materials with high luminous efficiency and good light output stability and a method of manufacturing the same.